Several methods are available to assay for the presence or concentration of a predetermined analyte, like an ion, in a test sample. These materials include wet phase and dry phase colorimetric assays, and assays based on flame photometry, atomic absorption photometry, ion selective electrodes (ISEs) and multiple liquid phase partitioning. Recently, the ion selective electrode method of analysis has been more widely used, especially in regard to automated systems, as improvements in ion selective electrodes have developed. In particular, ion selective electrodes now have sufficient selectivity, sensitivity and operating lifetimes to be useful in automated systems. See for example, U.S. Pat. Nos. 3,598,713; 3,502,560; 3,562,129; 3,691,047; 3,753,887; 4,839,020; 4,818,361; 4,743,352; 4,713,165; 4,810,351; 5,174,872; and 5,288,388.
The use of ISEs in the analysis and monitoring of charged as well as neutral species and gases has been continuously expanding. See for example, Ruzicka J. et al., Anal Chim. Acta, 62, 15-28 (1972); Ruzicka J. et al., Chr. Anal Chim. Acta, 67, 155-78 (1973); Hulanicki A., et al., Analust, 107, 1356-62 (1982); Stevens C. A. et al., Anal Chim. Acta, 248, 315-21 (1991). Even though the symmetric ISEs have found a wide range of applications they still have certain inherent limitations. They are mechanically complicated, and thus difficult to manufacture in small size; the internal solution increases the system impedance, and finally, due to internal compartment, they cannot withstand high pressures. See for example, Selig W., Anal Letters, 15(A3), 309-29 (1982); Cattral R. W., et al. Ion Selective Electrode Rev., Vol. 6, pp. 125-71 (1984); Nikolsky B. P. et al., Ion Selective Electrode Rev., Vol. 7, pp. 3-39 (1985); Cunningham L. et al., Analytica Chimica Acta, 180, 271-79 (1986).
In the past 10 years, various magnesium ion-selective electrodes (ISEs) have been described in the literature. See for example, Behm F. et al., Helv. Chim. Acta, 68, 110-118 (1985); Rouilly M. V. et al., Anal. Chem., 60, 2013-2016 (1988); Muller M. et al., Mikrochim. Acta, III, 283-290 (1988); Maj-Zurawska M. et al., Anal Chim. Acta, 218, 47-59 (1989); Maj-Zurawska M. et al., Anal Chim. Acta, 236, 331-335 (1990); Rouilly M. et al., Clin. Chem., 36, 466-469 (1990); Hu Z. et al., Anal Chem., 61, 574-576 (1989); Lewenstam A., Anal Proc., 28, 106-109 (1991); Spichiger U. E. et al., Fresenius J. Anal Chem., 341, 727-731 (1991); Spichiger U. E. et al., Magnesium-Bulletin, 13(4), 140-144 (1991); Spichiger U. E. et al., 2nd Bioelectroanalytical Symposium, pp. 185-211 (1992); Schaller U. et al., Pflugers Arch., 423, 338-342 (1993); O'Donnell J. et al., Anal Chim. Acta, 281,129-134 (1993); Eugster R. et al., Clin. Chem., 39, 855-859 (1993); and O'Donnell J. et al., Mikrochim. Acta, 113, 45-52 (1994).
A drawback to most of these electrodes is lack of satisfactory discrimination of magnesium in the presence of other divalent alkaline earth metals, notably calcium. Calcium ions are usually present in samples to be analyzed for magnesium ions, and in fact, the calcium ion concentration is often higher than the magnesium ion concentration. Thus, practical application of magnesium ISEs is often quite restricted. The present invention provides a buffer solution which eliminates the competition effects of calcium for the determination of magnesium concentrations in solution.